The rapid progress in modern telecommunication brings with it an increasing demand for a fast and efficient way to rout information between many users. As optical fibers replace old copper wires, a need for a low-cost direct optical switching is rising. Such optical switching components should operate within contemporary communication network systems that support Dense Wavelength Division Multiplexing (DWDM) for various applications, such as optical add drop multiplexing (OADM), optical cross connections (OXC), protection, restoration, etc.
In order to achieve higher capacity more communication channels are required. Therefore, a broader bandwidth of optical components is essential. Such broadband components must be fabricated in large numbers with low cost and high reliability. Reliable solid-state devices with no moving parts are suitable for mass production, as was demonstrated in the microelectronics industry. Planar lightwave circuit (PLC) technology is one of the most promising solid-state technologies, and a Mach Zehnder-Interferometer (MZI) is one of the most successful and useful structures built in this technology.
The MZI was invented more than century ago. It has been extensively used in the design of all-optical switches, filters, attenuators, etc. However, a standard MZI is not broadband, due to the high wavelength dependency of its couplers, and in some configurations, also due to the length difference between its two arms.
Few attempts to produce broadband MZI optical switches were made in the past. One approach, for thermo-optical silica-based switches, proposes Wavelength-Insensitive Couplers (WINCs) instead of directional couplers, where each WINC is actually a complete MZI by itself (Kitoh T. et al. “Novel broad-band optical switch using silica-based planar circuit”, IEEE Photon. Technol. Lett. 4, pp. 735-737, 1992). This device is very long and uses three electrical drivers and three active electrodes, thus having complicated control and consuming high electrical power. In addition, its Extinction Ratio (ER) performance is insufficient—17 dB over the spectral range of 1.25-1.65 μm.
Cohen et al. in U.S. Pat. No. 5,418,868 employed broadband adiabatic couplers (first described by Henry C. H. et al. “Analysis of mode propagation in optical waveguide devices by Fourier expansion”, IEEE J. Quantum Electron. 27 pp. 523-530, 1991) in their proposed MZI switch. At the core of the couplers are two closely adjacent waveguides of gradually varying widths, layed out so that the separation between the two is constant. These couplers and MZI switches have a few disadvantages: they must be very long in order to be adiabatic, and the coupling effect of the small (few μm) gap between the coupler's waveguides is very sensitive to fabrication, e.g. to over-etch, material stresses, etc., resulting in a deteriorated extinction ratio, or alternatively in a narrowed operational wavelength window. Under normal fabrication conditions, the ER over the spectral range of 1.25-1.6 μm is only 15 dB, which is also insufficient for most current applications.
Silberberg in U.S. Pat. No. 4,775,207 introduced 2×2 digital optical switches (DOS) via an electro-optic effect, and fabricated in materials such as Lithium Niobate (LiNbO3) with large electro-optic coefficients. The proposed DOS has an optical step-like response to the switching voltage. It is based on an asymmetric waveguide junction structure, composed from two input waveguides of unequal width, a wide central region and a symmetric output branching. In contrast with a MZI, a DOS has only two functional states, controlled by the electrical field. This allows the incoming optical signals to be routed to either one of the output ports (i.e. a “digital” response instead of the useful analog response of MZI switches).
Hwang et al. in a paper named “Polymeric 2×2 electrooptic switch consisting of asymmetric Y-junctions and Mach-Zehnder interferometer”, Photonics Letters, vol. 9, No. 6, June 1997 describe a 2×2 switch comprising a pair of couplers, each having two straight branches of different but constant widths, separated over a coupling length by a changing spacing therebetween. The branches blend in a symmetric intersection area. Because the branches are straight, the switch is by necessity large. Hwang et al. do not mention an extinction ratio, and it is unclear that the response is wavelength independent over a large range.
There is thus a widely recognized need for, and it would be highly advantageous to have a high-tolerance broadband MZI switch that does not suffer from the disadvantages of previous switches, as listed above, and which has in particular a wavelength independent response and higher ERs.